I know people do this to keep from being bumped up, and if the system worked as you describe (strikes only when the averaged dynamic rating exceeds the threshold) I'm sure they would do it to avoid strikes.

But that is my exact point, that because it is such an obvious loophole to avoid strikes, I doubt the USTA would implement it this way. A more reasonable way to do it is to have strikes occur when individual match dynamic ratings exceed a threshold. That way, it is harder to game the system.